High density optical recording systems which may be used for recording and playing back information are known in the prior art. For example, U.S. Pat. No. 4,097,895, entitled "MULTI-LAYER OPTICAL RECORD," issued on June 27, 1978, to F. W. Spong, relates to an optical disc record/playback system wherein data are recorded on the surface of a recording medium. In a Spong system the thermal energy of a focused high intensity light beam causes variations of the optical properties on the surface of the recording medium. For example, in one system the thermal effects of a laser beam form pits in an absorptive coating on the surface of an optical disc. In the Spong system, approximately 10.sup.11 bits of information can be recorded on one side of a disc-shaped record medium having a thirty centimeter diameter.
In a typical optical recording/playback system, there are four sources of motive power used for generating relative motion between the information-bearing surface of the optical disc and the optical system including the light source and detectors. First, a motor drives the turntable on which the optical disc is fixedly mounted to impart rotational motion of the disc relative to a fixed position. Second, a translation stage, a part of which is the subject of the present invention, moves a portion of the optical system, including the focusing objective lens, radially across the disc, generally perpendicular to the information tracks. Third, a galvanometer located on the translation stage deflects a mirror so as to align a light beam through the focusing lens onto a selected track. In general, the translation stage may be thought of as the coarse movement and the galvanometer as the fine movement with regard to directing the record or playback beam onto a particular information track on the disc. Finally, a focus actuator, which may be a voice coil located on the translation stage, moves the focusing objective lens in a direction normal to the information-bearing surface of the disc.
The design of the translation stage is influenced by the need for smooth motion which is quickly and precisely responsive to a control signal. It is typically driven by a motor. In one prior art system, a feedscrew is adapted to be rotated in response to rotations of a motor shaft. The feedscrew is in mesh engagement with a bearing surface of the translation stage. When the motor is activated and the shaft rotated, the feedscrew causes the translation stage to move inwardly or outwardly along a radius of the disc record, depending on the direction of rotation of the motor shaft.
More recently, typical recording/playback systems have employed a linear electric motor in the translation stage to provide smoother and more precise operation. Typically, an inductive winding is energized by a current directed so as to induce the desired motion of the moving member carrying the translation stage. The direct drive of a linear motor lessens the mechanical interaction between driving force and translation stage, thereby providing a potentially smoother excursion of the translation stage.
Of greater significance to the smoothness of the excursion is the quality of bearing surface on which the translation stage rides. Air-supported translation stages or slides are well known and are currently widely available commercially. In these cases, air is typically supplied to the moving member where it is distributed through a plurality of orifices disposed about its inner walls so that there is a continuous film of compressed air between the sliding member and the rail. It is easily seen that if the air were to be supplied through orifices in the rail, then those orifices not positioned in alignment with the sliding member would be fully open and the system would require a large volume of air. In addition, if air were to be supplied through orifices in the rail, then as the sliding member started to enter upon an orifice, an instantaneous perturbation to the sliding member would result.
Nevertheless, there are also several disadvantages of a translation stage slide apparatus which supplies compressed air from the sliding member. First, such an arrangement requires that the sliding member be constructed about the rail, and it is very difficult to effect such a construction with the clearances (approximately 10 to 15 micrometers) required to maintain the air bearing surfaces. Second, since pressurized air is supplied to the sliding member, the air hose attached thereto will cause perturbations by the varying amount of drag as the sliding member traverses the rail. Third, if a linear motor is to be used to induce motion, space has to be made available between the rail and sliding member, thus sacrificing some of the air bearing area. This third-mentioned disadvantage can be alleviated by implementing a combined repelling/attracting arrangement (e.g., pressurized air and vacuum) at the side of the rail opposite the linear motor, but this arrangement merely compounds the second-mentioned disadvantage by adding the drag of a second hose.